这题其实可以扩展一下：求连续出现3次及3次以上相同积分的情况————采用自连接（保证id是连续的即可），哈哈，做到举一反三！
select g1.number
from grade g1
where (select count(*) from grade g2 where g1.number = g2.number) >= 3
group by number;

或者参考：
select number from grade group by number having count(*)>=3;